Date of this Version
The presence of heavy metals within aquatic environments can be attributed to sources such as industry, municipal discharges, agriculture and natural processes which release various amounts into natural systems. Metals such as copper, zinc and chromium are listed as essential micronutrients for many plant and animal species. Of potential concern, however, is the tendency of heavy metals to accumulate within the food webs of ecosystems. Classified as biologically hard to degrade, heavy metals may concentrate to significant amounts within the trophic levels of a particular ecosystem. At these increased concentrations many of these heavy metals become toxic to living organisms.
Previous research had detected the presence of heavy metals in water samples collected from the Missouri River near Sioux City, Iowa. Levels of zinc, copper and lead exceeding state quality standards water were observed in 1984-85 (Tondreau, 1984-86). The direct source of the metals was unknown but two local industries involved in metal-plating were suspected. Around this time the secondary treatment phase of Sioux City's wastewater treatment plant was shut down due to a shock load of zinc. The high metal levels were impetus to conduct studies on the bioaccumulation of metals within the aquatic communities found in the main channel border habitat of the channelized Missouri River. Conducted during the years 1986 and 1987, concentrations of copper, chromium, lead and zinc were measured in water, fish, periphyton and invertebrates collected at two sites just upstream from Sioux City R.M. 735 and downstream from Sioux City at R.M. 722. (Shane and Tondreau, 1987). Bioconcentration factors (BCF) were calculated by dividing the metal concentration of each aquatic group by the metal concentration in the water. Bioconcentration factors for the aquatic communities of periphyton, invertebrates and fish for 1987 are listed in Table 1.